Put the tube on the broomstick. Use automotive spray paint. Always perform a first test spray to unblock
the nozzle, far from the tube. Then spray onto the top of the tube, at a distance of 30 cm or so,
in FINE coat. Never try to cover the undercoat with only 1 paint coat.

Cover the tube then wait for the paint is wet (6 hours for example). Then rotate the tube to paint
another part. Once all of the tube is painted, the undercoat should remain visible. 3 or 4 layers are required.
Be patient, with a 10-inch scope it's a one-week long work.

Carefully clean a patch of adhesive black velvet (vacuum-cleaner, clothes brush, large adhesive tape)
then stick it front of the eyepiece holder (and, if you want, as a large ring just above
the primary mirror, but I don't like it because the velvet tend to hold -then release- dust. Now
that's REAL matte black!